1. Field of the Invention
This invention relates to a magnetic disk apparatus in which a magnetic head is moved on a rotating magnetic disk and to a method for controlling the magnetic head.
2. Background Art
In a control system for positioning a magnetic head of the magnetic disk apparatus, a digital control system using microcomputer is generally composed. That is, control command is calculated inside a microprocessor from position information of the magnetic head that is discretely obtained, and an activation drive of an actuator is provided with the control command through D/A (Digital-Analog converter). In general, because an actuator has mechanical resonance in a high-frequency band, it is very important that a feedforward control input that does not excite mechanical resonance is generated in order to move the magnetic head to a target position with high speed, low vibration, and low noise.
As a method for moving the magnetic head by a short distance at high speed, it is thought that the feedforward control input for the actuator so that the mechanical resonance is not excited and a target position command to a feedback control system are preliminarily calculated by an optimization method and held as a table. However, it is impossible from the memory capacity of microprocessor to adopt such a method with respect to every seek distance. Therefore, in the case of long distance seek, the feedforward control input for the actuator and the target position command have to be generated online.
As a method therefor, there can be thought a method for, providing an actuator model inside the control system, making the model velocity comply with the target velocity curve, and thereby providing the feedback control system with the control command to the model and the model position as the feedforward control input to the actuator and the target position command respectively (JP-A 9-73618 (Kokai) (1997)). However, in the head positioning control system described in JP-A 9-73618 (Kokai) (1997), if position and velocity of the model are not near the actual position and velocity of the magnetic head, the magnetic head comes to overshoot in settling or the like. Therefore, some model modification becomes required in the seek.
As a method therefor, there is a method for, updating the model by adding the feedback control output to the input of the model, estimating position and velocity of the magnetic head by the model, and composing the velocity control system by using the estimated position and velocity of the model, in the seek first half (for example, The 74th JSME (Japan Society of Mechanical Engineers) Spring Annual Meeting, Vol. 4, pp. 410-411 (1997) (in Japanese), FIG. 3). In this method, further, in the seek second half in which the magnetic head comes near the target position, the feedback control output is switched to be added to the actuator, and thereby, a general two-freedom-degree control system is composed.
By the way, when a servo system has to be composed like the magnetic disk apparatus, the feedback controller has an integrator. Therefore, when such a seek control system is composed, output of the integrator is also added to the model and therefore the output of the integrator becomes large when an external force such as magnet latch force is large. Therefore, in switching to a general two-freedom-degree control system, transient response comes to occur in the control command.
Moreover, in the case of the magnetic disk apparatus, when the magnetic head moves at high speed, servo pattern becomes diagonally crossed and position detection noise becomes large. The position detection noise influences the velocity feedback control system of the model side through the feedback control output, and therefore, vibratory feedforward control command becomes generated. This can also cause quack. From these things, in the control system described in the JP-A 9-73618 (Kokai) (1997) or The 74th JSME Spring Annual Meeting, Vol. 4, pp. 410-411 (1997), FIG. 3, it is difficult to realize robust seek control in the environment in which a large external force works. Furthermore, it is also difficult to reduce the influence of the position detection noise.